1. Field of the Invention
The invention pertains generally to automatic temperature control systems for the passenger compartments of vehicles and is more particularly directed to a closed loop modulator control circuit for regulating the response of such systems.
2. Prior Art
The most widely used temperature control systems for vehicles today are those with a manual temperature control selector and a manual blower or fan control selector. An operator by selecting differing combinations of positions for these controls can achieve a reasonable degree of comfort in the passenger compartment during most ambient conditions requiring heat.
The problem with such systems is their inconvenience because of the necessity for constant readjustment. When initially bringing the passenger compartment up to temperature in cold ambient conditions, the normal person will use the highest heater setting and highest blower speed or wait longer than need be for comfort. As the vehicle warms at the maximum output of the heater and the desired temperature of comfort is exceeded, a readjustment of blower speed and heater setting is attempted. Thereafter, readjustment of the combined settings is a conjecture on the part of the operator where the final temperature of the compartment will stabilize. If he, as most operators do, guesses wrong, further adjustments will be tried until a satisfactory temperature is reached.
To answer the need for a more facile method of providing comfort to passengers of automotive vehicles than before, automatic temperature control systems have been devised. One particularly advantageous system has air ducting through which an air flow is directed by means of a blower. The ducting is provided with a diverter box which proportions the air flow between a bypass duct passage and a heater passage containing a heater core. A variably positioned blend air door proportions the quantity of air passing into either the bypass duct or the heater duct so as to control the temperature of the air flowing out through the ducting. The ducting vents into the passenger compartment and further provides for the recirculation of the passenger air and mixing with some outside air for ventillation.
The temperature control is carried out by means of a manual temperature selector device such as a rheostat and a passenger compartment temperature sensor such as a thermistor. The thermistor and rheostat produce electrical control signals (resistance changes) corresponding to the sensed temperature level and the desired temperature level respectively. Upon development of a temperature error signal indicating a difference between the selected temperature and the actual temperature, regulation of the position of the blend door is initiated.
The positioning is realized by applying the error signal to a vaccum modulator device which generates a vacuum pressure level corresponding to the error signal. The vacuum level is fed to a vacuum operated actuator which then positions the blend air door. The system variables are calibrated such that the change in position of the blend air door increases or decreases the temperature of the air flow within the ducting system so as to correct the temperature level in the passenger compartment.
For a sensed temperature above the selected temperature level, the vacuum level is adjusted to cause the blend air door to assume a position whereby more air is directed through the bypass into the passenger compartment. Conversely, if the sensed temperature level is below that selected, the blend air door position is adjusted to increase the temperature of the air passing into the passenger compartment to correct the temperature differential. The degree of error signal of either sense determines the extendt of movement of the blend air door in the direction tending to correct the temperature difference condition.
The vacuum modulator for this system provides the vacuum level in response to the input signals of a modulator control circuit. The modulator has an interior chamber that is ported to communicate with a vacuum source and a vent source. Each port is closed by a valve means attached to the end of a bimetal arm around which is wound a heater coil. Opposite each valve means, an interrupter contact is positioned such that when a particular bimetal arm is deflected the contact will close. The modulator control circuit changes the vacuum level by energizing the heater coil of the arm associated with the vacuum source, if a greater level is desired, or energizing the heater coil of the arm associated with the vent source, if a lower vacuum level is desired. Control of the current levels in the heater is provided by pulsing the coils with current in response to the interrupter contacts making and breaking as the bimetal arms open and close the ports.
Although superior in performance, for an automatic temperature control system to compete effectively against the present manual system, it must also rival the manual system in simplicity. Thus, the reduction of circuit complexity in the modulator control circuit while still maintaining adequate control is necessity. Prior modulator control circuits have neither optimized closed loop response nor effected the desirable goal of circuit simplification. Therefore it would be advantageous to provide a modulator control circuit which acheives both of the aforesaid objectives.